Aircraft fire detectors conventionally utilize elongated, distributed thermistor elements which extend through the aircraft for temperature monitoring purposes. Such a thermistor element typically comprises a small diameter tube filled with a thermistor material, in which is embedded a wire running the full length of the interior of the tube. Such thermistors exhibit a resistance characteristic between the tube and the wire which decreases with increasing temperature. By design, a fire occurring at any point along the distributed thermistor will cause a substantial reduction in the resistance measured between the wire and the tube at either end of the thermistor, and this reduction in resistance can be utilized to detect and signal the occurrence of a fire, followed by automatic shut down of the aircraft engine and other systems.
False alarms have always been a significant concern in such systems, not only because of the in-flight shut down resulting from a false fire warning, but also because aircraft equipment indicating a fire warning must be removed from service, and substantial delays are encountered in testing the equipment before it can be restored to service.
In order to cope with the problem of false fire alarm indications, such fire detection systems have utilized two thermistors over the same path, thereby introducing redundancy. By requiring the same alarm condition to exist on both thermistors before an alarm is issued to the cockpit crew, such redundant systems reduce the incidence of false alarms. With this type of system, the defective thermistor loop may be shut down, and the aircraft may be dispatched with only a single loop operative. However, whenever such single loop dispatch is implemented, the risk of a false warning occurring is substantially increased.
It is therefore an object of the present invention to minimize or eliminate the occurrence of false fire alarms in aircraft fire detecting systems and to minimize or eliminate the incidence of single loop fire detection system operation.
In accordance with the present invention, the resistance measurements performed on the redundant thermistor loops are differentially monitored for abnormal differences between the two identical thermistor devices which are essentially exposed to the same operating conditions. Such differences can be an indication of deterioration of the condition of one of the sensors. Since it normally takes many hours, if not days, weeks or months for the deterioration to reach the alarm level, the present invention provides a very early indication of impending failure. By displaying signals regarding this deterioration to maintenance personnel, it becomes possible to fix the problem before a failure ever occurs.